Solid state cameras, which have been popular for many years in scientific, military and other specialized fields, are becoming increasingly popular for general consumer use. This increased popularity has come with improvements in technology and reductions in camera cost.
Solid state cameras include many components that cooperate to capture an optical image for storage in an electronic format. For example, a solid state camera typically includes an array for converting an optical signal to an electrical signal, an optical system for focusing an image onto the array, processing circuitry for processing the image, and memory for storing the image. The camera may also include a display for viewfinding and playback functions.
Several different types of solid state cameras are known. These generally differ in the type of array used in the camera. Arrays are integrated solid state devices, and are typically fabricated on a semiconductor substrate. The two most popular types of arrays are charge coupled device (CCD) arrays and complementary metal oxide semiconductor (CMOS) arrays. Each of these arrays typically includes a large number of individual image-capture elements, or pixels, arranged in a two-dimensional pattern of rows and columns. Each image capture element includes a photodiode or other photodetector that, when illuminated, produces an electric charge proportional to the intensity of the illumination. The electric charge from each image capture element is then processed to form the digital image.
CCD arrays are the oldest and most commonly used type of solid state array. The term “charge-coupled device” describes how a CCD array moves the charges produced at each pixel into the downstream processing and memory circuitry. Besides the array of light-sensitive pixels, a CCD array also includes at least one horizontal row, or register, of non-light sensitive conductive elements positioned adjacent an outermost row of pixels. After exposure of the array to an optical signal, the charges in the outermost row are simultaneously moved to the horizontal register, and the charges in all of the other rows simultaneously shift one row toward the horizontal register. The charges in the horizontal register are then moved in serial fashion to downstream processing circuitry, where they are converted into digital format. These steps are repeated until all charges have been read from the array.
Different sub-types of CCD arrays may utilize various modifications to the basic CCD method of operation to improve camera performance. For example, two types of CCD arrays, known as frame transfer arrays and interline transfer arrays, include an array of storage cells located adjacent the array of pixels, wherein the array of storage cells includes one storage cell for each pixel. After exposure, the charge from each pixel is transferred into its corresponding storage cell. Then, the charges are moved from the storage cells into the processing circuitry in typical CCD fashion. The two types of arrays differ in the positioning of the storage cells relative to the pixels.
CMOS arrays, though developed more recently than CCD arrays, have become increasingly popular due to several advantages they offer over CCD arrays. For example, compared to CCD arrays, CMOS arrays can operate at a lower voltage and consume less power. Also, the pixels in a CMOS array are individually addressable. Furthermore, because CMOS arrays may be manufactured utilizing standard CMOS technology, active circuit elements may be integrated into each pixel, allowing pixel-level image processing. This also allows ordinary CMOS fabrication installations to be used to make the CMOS image sensors, allowing for improved economies of scale compared to the manufacture of CCD arrays.
Although current CMOS systems offer improvements to CCD systems in some areas, CMOS systems also share some common drawbacks with CCD systems. For example, CMOS arrays cannot begin a new image capture cycle (“integration”) until the charge from the previous integration has been read from each image capture element and processed. This is typically done in serial or partially parallel fashion. Some cameras have buffers to hold data from a few exposures before processing, but these are generally too small to hold many exposures. Moreover, each pixel cannot be read and processed until the previous pixel has been read, processed and stored. Thus, the transfer, processing and storage of data acts as a bottleneck that slows down the capture of subsequent images. This delay may be undesirably long for applications such as video, or for cameras with a large number of pixels.